Clinical instruments for IOP measurements are referred to as ophthalmic tonometers or just tonometers. Tonometric procedures are based on creating a deformation of the cornea with a known external force and then relating the extent of the deformation to the IOP with appropriate previously obtained calibrations using enucleated eyes or model eyes. Deformations have been historically grouped into indentations and applanations; the latter are deformations of lesser extent in the volume of ocular fluid being displaced and therefore cause a smaller amount of stretching in the ocular coats (sclera and cornea), which stretching causes inaccuracies in the IOP determinations and is referred to as ocular rigidity. A brief history of tonometers actually in use at some point in time, can be found in textbooks or review articles (e.g., Craven, E. R., Shields, R. L.: "Tonometry in Clinical Practice", in the Glaucoma Series, ed.: Starita, R. J., Clinical Signs in Ophthalmology, vol XII, no.1, Mosby-Year Book, Inc., 1990).
The contact tonometer most widely used at the present time is the applanation tonometry devised by H. Goldmann (U.S. Pat. No. 3,070,997) in which the cornea is flattened by a device having a plane contact surface; in it the force necessary to achieve a standardized applanation 3.06 mm in diameter is calibrated to provide the IOP. For a typical cornea having a radius of curvature of 8 mm this corresponds to a central flattening of dX=0.15 mm; the corresponding volume of the displaced intraocular fluid is about 0.2 .mu.l or approximately 3.times.10.sup.-5 of the entire intraocular fluid volume and is achieved typically with a force of only 1 to 2 g.
The Goldmann applanation and other contact-type tonometers suffer from a number of shortcomings. Their use requires the cornea to be prepared by applying a topical anesthetic to minimize patient's discomfort. This, however, increases the risk of eye damage because the patient's sensitivity to normal pain is depressed. Further, any contact with the corneal tissue carries the risk of infection and corneal abrasion. The results of measurements depend on the analysis of deformation produced by mechanical contact with the cornea which necessitates that the morphology of the cornea be close to an ideal condition; however, this is not the case in many corneas altered by pathologic conditions, thereby precluding the use of Goldmann tonometry. Finally, the accuracy is adversely affected by size variations of eyes and by stiffness variations of the cornea and sclera, uncontrolled movement of the patient's head and eyelids, and the cardiac and pulmonary cycles registered through the corneal surface. Accordingly, these instruments require great skill in order to provide safe and accurate measurements and must be used either by a physician or a well-trained clinical technician.
The most widely used non-contact tonometer at the present time is the air puff instrument originally introduced by Grolman in the 1960's (U.S. Pat. No. 3,538,754). In this type of tonometer, a pulse of compressed air is directed at the cornea which will be deformed from convex to flat and then to concave following the increasing pressure of air; an optical system identifies, in time, the applanation event and, being synchronized with the air puff source, can indicate the IOP. Several improvements of the Grolman air puff tonometer have also been patented. While the air puff type tonometers have eliminated both the need for the topical anesthesia and contact with the eye, they are expensive, bulky, show a gradual decrease in accuracy with increasing pressure to be measured, difficulties with alignment while the audible sound and strong surge of air hitting the surface of the cornea cause patient apprehension and discomfort.
Several designs of contemporary tonometers disclosed in numerous U.S. patents issued in the 1980's and 1990's utilize different techniques based on combinations of sound, ultrasound, optics, and electronics.
U.S. Pat. No. 5,396,888 issued in 1995 to N. Massie and B. Maxfield discloses a combination of multiple ultrasonic sources with optical devices in an attempt to create an improved tonometer. That tonometer is based on an analysis of the deformation of the cornea caused by a known external force. The use of ultrasound by the '888 patent to generate the force applied to the eye has definite advantages over air-puff tonometry where compressed air is used. To increase the accuracy of the IOP measurements, this patent discloses ultrasound power generation, measuring and ranging transducers, a visualization system to provide an image of the eye for the clinician, applanation and indentation sensors, and a transverse alignment indication and detection system. In addition, to reduce the sensitivity of the instrument to various sources of electromagnetic interference and to the patient's head and/or eye movements, complex phase sensitive modulation-demodulation techniques were incorporated. These highly complex and expensive additions restrict the potential use of such an instrument to clinics, hospitals and research laboratories.
The complexity and numerous shortcomings of non-contact tonometers proposed so far are part of the reasons why they have not found a real commercial use and even routine eye pressure checkups still involve expensive contact equipment, requiring topical anesthesia and specially skilled persons (usually a physician) to be used safely and accurately.
Since the most important application of ophthalmic tonometry is its use for preventive or early detection of glaucoma and for conducting the ongoing treatment of diagnosed glaucoma by establishing that the IOP stays inside safe limits through regular tonometric monitoring, there is a need for a reliable, accurate, simple-to-use, safe and inexpensive tonometer affordable for daily self-use by any individual at risk for glaucoma.